Method of permanently marking polytetrafluoroethylene



p 16, 1963 c. L. FRIESE 3,085,912

METHOD OF PERMANENTLY MARKING POLYTETRAFLUOROETHYLENE Filed Feb. 24,1960 POLYTETRA-FLUOROETHYLEN E INSULATED CONDUCTOR INVENTOR.

CHARLES L. FRIESE ATTORNEY 3,tl85 912 RETHOD F PERMAlNENTLY MARKHNGPOLYTETRAFLUQRQETHYLENE Charles L. Friese, Towson Township, HarfordCounty,

Md, assignor to Martin-Marietta Corporation, a cor-' poration ofMaryland Filed Feb. 24, 1960, Ser. No. 10,580 Claims. (Ci. ll72l2)Teflon or polytetrafluoroethylene has a wax-like surface texture whichcannot be permanently marked with conventional marking inks in asatisfactory manner. Hotstamping produces a permanent mark but causesthe Teflon to break down, which is highly undesirable in cases where theTeflon is used as an electrical insulation since the stamping processreduces the insulating capacity of the Teflon. Heretofore, Teflon-coatedelectrical conductors have been marked in a non-permanent manner, suchthat the markings may be rubbed off easily or become illegible duringuse. It has been considered necessary in many instances to admix anidentifying color with the Teflon prior to polymerization so that thedifferent types of wire might be identified by the distinctive coloringin the Teflon. This method of identification has proven unsatisfactory,however, because it requires a large inventory of wires of differentcolors to be maintained in stock. Another method of marking Teflon Wirecomprises placing ink striping on the surface of the Teflon insulationand then passing the inked Teflon wire through a gas heated oven to drythe ink. The dried striping, however, is not permanently affixed to theTeflon and may be readily rubbed off.

It is a purpose of this invention to provide a method of identifyingTeflon wire by permanently marking same with ink in such a manner thatthe electrical qualities of the Teflon are not materially impaired. Ithas been discovered, in fact, that the present marking method actuallyimproves certain electrical qualities of the Teflon coating oncommercial Teflon wires.

In outline, as shown on the drawing, the present method comprisesimpressing Teflon ink on the Teflon wire and then rapidly heating thesurface of the impressed Teflon wire with radiant energy to fix the inkmarking permanently. The markings so produced are extremely durable andcannot be removed except by destroying the underlying Teflon.

This method differs essentially from all prior marking methods in thatthe Teflon insulation is heated by means of radiant energy, whereas,heretofore, the Teflon has been heated by means of convection orconduction. The conduction method is exemplified by hot stamping, which,as noted above, materially decreases the electrical insulating capacityof Teflon. Convection heating, utilized in applying striping to Teflon,cannot be performed at a temperature which would cause the ink to bondpermamanently to the Teflon insulation because the temperature of thewire conductor would be raised to excessive values, such that theindividual strands of conductor are caused to stick together, therebyseriously reducing the flexibility of the Teflon wire. For example, whenstriping is dried in a gas heated oven at about 850 F., the silverplated individual conductor strands become joined.

In the present process, heat is transferred to the outer surface of theTeflon insulation at a high rate for a short period of time, the rate atwhich heat is absorbed at the outer surface of the Teflon insulationbeing much greater in magnitude than the rate at which heat is conductedfrom the outer surface to the wire conductor. In

Patented Apr. 16, 1963 ree this manner, a steep temperature gradient isestablished radially in the Teflon insulation during the heating step.Before an excessive amount of heat is able to build up in the interiorof the Teflon wire, the wire is cooled. Depending on the radiant energyheat flux, the time of exposure and the physical dimensions of theTeflon wire, cooling may be effected by either allowing the processedwire to stand in air or by cooling the wire with a forced draft of airor by other suitable accelerated cooling means.

For the purposes of the present invention, the term Teflon encompassesnot only the homopolymer of tetrafluoroethylene but also substanceswherein an organic material is admixed with or copolymerized withtetrafluoroethylene in such proportion and in such manner that theresulting admixture or polymeric product partakes of the properties oftetrafluoroethylene. The organic additive may constitute up to about 25%of the admixture or copolymeric product and may, for example, be amaterial like polyisobutylene; butyl rubber including the elastomericcopolymers of isobutylene and diolefins such as butadiene; polyacrylatesand polyalkylacrylates including polymethylacrylate, polymethylmethacrylate, polyethylacrylate, polyethyl methacrylate and the like;butadiene-acrylic copolymers including butadiene-acrylate,butadiene-acrylonitrile, and butadiene-acrylamide; butadiene-styrenecopolymer; plasticized polystyrene; polyvinyl halides and polyvinylidenehalides including polyvinyl chloride, polyvinyl fluoride, polyvinylidenechloride, and polyvinylidene fluoride; and alkyl-acrylate copolymersincluding copolymers of methyl methacrylate-10% methylacrylate, 90%methyl methacrylate- 10% ethylacrylate and the like. The term Teflonwire, therefore, is to be understood as a metal conductor having asheath of the homopolymer of tetrafiuoroethylene or a sheath of any oneor more of the admixtures or copolymeric products hereinbeforeenumerated. Teflon ink, in an analogous fashion refers to liquidsuspensions or thermoplastic materials comprising a suitable pigment orcolored filler admixed with particles of tetrafluoroethylene orparticles of any one or more of the aforesaid admixtures or copolymericproducts. Specifically, the composition of such ink includes (1) apolytetrafluoroethylene dispersion; (2) a ceramic oxide pigment; (3) abinder (or thickener); (4) a distilled water carrier; and (5) afluorocarbon wetting agent. Ink in accordance with this specification iscommercially available from the Hi- Temp Wire Company as Black MarkingFluid, Code 913A. This invention, however, is not directed to theparticular materials used; but, rather, to the method by which Teflonwires of the type described may be permanently marked with Teflon ink.

In accordance with the present invention, the Teflon Wire or otherTeflon article is marked with the desired identifying symbol by pressingTeflon ink on/ or into the Teflon surface. For this purpose, anyconventional printing machine may be used. A particular machine whichhas been employed successfully for marking Teflon wire is the DualPrinter made by Duncan M. Gillies Co., Inc., West Bozleston,Massachusetts, which impresses the Teflon wire by opposed printing discsthrough which the wire is fed. Each disc is provided on its peripherywith a plurality of raised symbols and is made to pass through a bath ofTeflon ink whereby the raised symbols become coated therewith.

The Teflon wire is subjected on its marked surface to a predeterminedflux of radiant energy, the principal Wavelengths of which are ininfrared region, which is absorbed by and heats the outer surface of theTeflon wire. Heating is continued for a length of time sufficient topermanently bond the ink to the Teflon, whereupon heating isdiscontinued to prevent an undue buildup of heat in the interior of thewire as would adversely affect the Teflon insulation or the wireconductor.

The source of infrared energy may be a resistance element, a quartzinfrared lamp or other conventional device. In the preferred embodimentof the present invention radiant energy is supplied by a metalresistance element of cylindrical configuration through which the wireto be heated is passed. The resistance element is suitably insulated bymeans of a refractory material so as to form an open ended furnace.

After processing all samples were subjected to the insulation resistancetest, the dielectric strength test, the cold bend test and the heatresistance test as detailed in National Aircraft Standard 703. Anunprinted sample which had not been heated was included as a controlspecimen for comparison.

The dielectric strength test calls for the application of 3000 volts, toClass A insulation for a period of five minutes without breakdown. Inorder to obtain a figure of relative merit for these specimens, thestandard dielectric strength test was followed immediately by adielectric breakdown test.

Results of these tests on 16 ga. Class A Teflon wire are listed in TableI.

Table I.Results of Infrared Irradiation of Class A Wire at IndicatedTemperature of 1800" F.

Insulation Dielectric Cold Bend Heat Resistance Feed Rate DurabilityResistance Sample No. (ft./sec.) ofPrinting (Megs/50 it.) Strengthbkdn.. Strength bl:dn., Strength bkdn,

(kv.) Orv.) (kv.)

Fair 320,000 Passed. 12.3 Passed 14.3 Passed" 13.5 Good 250,000.--.do.-- 10.5 ....do.... 15.0 ....do. 8.8 ....-do..... 2,000,000...-d0.... 10.1 ..do.... 12.3 ..d0.... 8.8 Excellent... 2,000,000....d0.... 10.1 .--.do.... 11.3 ...-do.... 8.4 Fall 2,000,000 ....do....10.8 12.5 ...-do.... 11.8 200,000 1o.. 11.8 .-.do.... 9.8 ..do.... 12.0

In order to provide a better understanding of the present invention,reference is made to the following specific examples, wherein the sourceof infrared energy was a heated Nichrome screen, 30 x 30 mesh, made of0.014 inch wire. The screen was formed into a tube 3 inches in diameterby 3 feet long, and insulated by a wrapping of Thermoflex, an asbestossheet manufactured by Johns- Mansville Co., Chicago, Illinois.

The screen temperature was monitored with a 22 ga. Chromel-Alumelthermocouple placed between the Nichrome screen and the insulationmidway of the tube. The particular Teflon ink used in the followingexamples was Black Marking Fluid, Code 9l3-A produced by Hi- Temp, Inc.,Chicago, Illinois.

EXAMPLE I Five 100 foot samples (A thru E) of #16 ga. Teflon insulatedwire, satisfying military standard MIL W 16878, were printed withHi-Temp black marking fluid (Code 913A), a Teflon ink, and sintered atspeeds between 0.3 and 0.7 ft. per second.

The screen temperature was manually controlled at a temperature of 1-800F. i20.

The following procedure was followed in bringing the furnace tooperating temperature. Four hundred amperes at 10 volts were supplied tothe Nichrome screen until the temperature indicated at the thermocouplewas 155 0 F., at which point current was reduced to 300 amperes. Whenthe indicated temperature reached 1800 F., the current was furtherreduced to 250 amperes. A

The optimum wire feed rate through the 36 inch furnace at 1800 F. was0.4 ft. per second.

Generally, the processed specimens exhibited marked improvements inbreakdown voltage following the cold bend test and in insulationresistance. However, the processed samples showed slightly lowerresistance to dielectric breakdown after having been subjected to thedielectric strength test and the heat resistance test.

In all instances it was impossible to remove the markings afterirradiation except by destroying the underlying Teflon.

EXAMPLE II One hundred foot samples of #22 ga. Teflon insulated wiresatisfying military standard MIL W 16878 were printed with Hi-Temp BlackMarking Fluid, Code 913A.

Three samples were processed at 1800 F. at speeds of 0.40, 0.35, and0.30 ft. per second, respectively, which are within the range of speedsat which optimum results were obtained with the #16 ga. Teflon insulatedClass A" wire of Example I. The marking on one of these samples had poordurability, and the insulation on the other two samples separated andburned. These failures apparently were caused by overheating.

Three additional samples were then processed at the same temperature,1800" F., but at higher feed rates, viz., 0.5, 0.6 and 0.7 ft. persecond, respectively. The markings after processing proved to be of fairto good durability. The temperature of the Nichrome screen was thenraised to an indicated 1950 F. Results are listed in Table II.

Table II.--Results of Infrared Irradiation of "Class C Wire at IndicatedT emperature of I 950 F.

Insulation Dielectric Cold Bend Heat Resistance Feed Rate DurabilityResistance Sample No. (IL/sec.) ol'Irinting (Megs/50 it.) Strengthbkdn., Strength blrdn, Strength hkdn.,

(kv.) (kv.) (kv.)

Passed.. 10.9 Passed.. 12.5 PassecL. 11.3 (10.... 9.0 ..--do.... 12.6.-..do.... 11.4 (10-... 11.7 ..do.... 12.0 d0.... 13.8 -..-do.... 13.5..-.d0.. 11.2 (10.... 8.0 .--.do.-.. 12.8 Cracked .-....(l0... 14.3

forced-air cooler placed at the exit end of the furnace was then turnedon, and the printed Teflon was fed through the furnace substantiallycoincident with the longitudinal axis thereof.

Specimens A through E of Example II provide a complete spectrum ofprocessing speeds with results ranging from failure due to insufiicientheating (at 0.9 ft./sec.) to unsatisfactory results, which were causedby excessive heating (at 0.5 ft./sec.) and consequent decomposition ofthe Teflon. Any speed between 0.6 and 0.8 ft. per sec. yielded excellentdurability at this furance temperature.

Decomposition of the Teflon was indicated by the color of the processedspecimen, namely, an oyster white coloration.

Results of the tests performed on the #22 ga. Teflon insulated wirefollowed the trend observed in Example I.

Although the invention has been described with particular reference tospecific embodiments thereof, it will be understood that it issusceptible of embodiment in many other forms, such as will appear toone skilled in the art in view of the foregoing teachings, withoutdeparting from the scope of the invention as set forth in the followingclaims:

What is claimed is:

1. A method of marking articles of homopolymers and copolymers oftet-rafluoroethylene comprising the steps of printing the surface ofsuch an article with ink in the desired pattern, said ink comprising apigment admixed with particles of polytetrafluoroethylene, andirradiating said surface with infrared radiation so as to cause said inkto become permanently bonded to said surface.

2. A method of marking polytetrafiuoroethylene coated wire comprisingthe steps of impressing ink onto the surface of said Wire, said inkcomprising a pigment admixed with particles of polytetrafluoroethylene,and irradiating said surface with infrared radiation so as to bond saidink permanently to said surface.

3. The method of claim 2, wherein said polytetrafluoroethylene coatedwire is cooled after the irradiating step.

4. In the method of marking polytetrafluoroethylene coated wire whereinthe surface of said wire is coated with ink so as to form an identifyingsymbol thereon, said ink comprising a colored filler admixed withparticles of a copolymer of tetrafluoroethylene, the improvementcomprising irradiating said surface with infrared radiation so as tocause said ink to become permanently bonded to said surface.

5. The improvement of claim 4 wherein said polytetrafluoroethylenecoated wire is cooled after the irradiating step.

References Cited in the file of this patent UNITED STATES PATENTS KreidlMar. 31, 1953 Roberts Feb. 11, 1 958 Osdal Aug. 29, 1961 FOREIGN PATENTSGermany May 24, 1956

1. A METHOD OF MARKING ARTICLES OF HOMOPOLYMERS AND COPOLYMERS OFTETRAFLUOROETHYLENE COMPRISING THE STEPS OF PRINTING THE SURFACE OF SUCHAN ARTICLE WITH INK IN THE DESIRE PATTERN, SAID INK COMPRISING A PIGMENTADMIXED WITH PARTICLES OF POLYTELRATUOROETHYLENE, AND IRRADIATING SAIDSURFACE WITH INFRARED RADIATION SO AS TO CAUSE SAID INK TO BECOMEPERMANENTLY BONDED TO SAID SURFACE.